Adaptive M-ary Quadrature Amplitude Modulation Research Articles

Performance Improvement of Non-Regerative Cooperative Relay Networks with Optimal Power Allocation and Adaptive M-QAM/M-PSK Modulation Over Generalized Fading Channels

This paper analyzes the performance of Channel Side Information (CSI)-assisted cooperative amplify-andforward (CAF) relay networks that employ both the node placement (i.e., relay position) based optimal power allocation policy among collaborating nodes and adaptive M-ary quadrature amplitude modulation (M-QAM)/ M-ary phase shift keying (M-PSK) techniques in generalized wireless fading environments. In particular, we advocate a simple yet unified numerical approach based on the marginal moment generating function (MGF) of the total received Signal to Noise Ratio (SNR) to derive analytical expressions for the average bit error rate (ABER), mean achievable spectral efficiency, and outage probability performance metrics. The proposed analytical framework is sufficiently general and flexible to characterize the performance of adaptive-link CAF relay networks over a wide range of fading distributions (i.e., not restricted to Rayleigh fading or independent identically distributed (i.i.d) Nakagami-m fading ) with independent but non-identically distributed (i.n.d) fading statistics across the spatially distributed diversity paths. Additionally, we further simplify the computational complexity, by employing the use of an “approximate MGF expression” to compute the system performance metrics over the generalized fading channel. Employing the above novel approach based on “approximate MGF” in conjunction derived analytical frameworks allows us to simplify the computation complexity of achievable spectral efficiency as well as ABER of CAF relay system in the generalized fading environments by simply replacing appropriate single channel MGF which is readily available.

Performance Analysis of the Link-Adaptive Cooperative Amplify-and-Forward Relay Networks with Opportunistic Relaying Strategy

This paper analyzes the performance of cooperative amplify-and-forward (CAF) relay networks that employ adaptive M-ary quadrature amplitude modulation (M-QAM)/M-ary phase shift keying (M-PSK) digital modulation techniques in Nakagami-m fading channel. In particular, we present and compared the analysis of CAF relay networks with different cooperative diversity and opportunistic routing strategies such as regular Maximal Ratio Combining (MRC), Selection Diversity Combining (SDC), Opportunistic Relay Selection with Maximal Ratio Combining (ORS-MRC) and Opportunistic Relay Selection with Selection Diversity Combining (ORS-SDC). We advocate a simple yet unified numerical approach based on the marginal moment generating function (MGF) of the total received SNR to compute the average symbol error rate (ASER), mean achievable spectral efficiency, and outage probability performance metrics.

Scaled Selection Combining for Adaptive Decode-and-Forward Relaying in Dissimilar Rayleigh Fading Channels

This paper enhances the conventional scaled selection combiner (SSC) for decode-and-forward (DF) relay networks using adaptive M-ary quadrature amplitude modulation (M-QAM) to improve the spectral efficiency. Compared with the conventional SSC designed for the combining of identically distributed diversity branches using the same modulation level, the improved SSC allows all diversity branches to choose different modulation levels according to the dissimilar channel conditions. Different scale factors are used for all diversity branches to reflect not only the performance degradation caused by possible erroneous relaying but also different error-resistance abilities of different levels QAM. We derive the bit-error-rate (BER) expressions for DF relay networks using SSC in a recursive way, with all channels conforming to independently and non-identically distributed (i.ni.d.) Rayleigh fading. Newton’s method is employed to obtain the numerical solutions of the optimal scale factors minimizing the BER, and the approximations of the optimal scale factors are derived in closed form for high SNRs. Theoretical analysis and simulation results show that the improved SSC can effectively combine diversity branches with different modulation levels, and for a DF cooperative network with N relay nodes, SSC achieves the full diversity gain of N+1 if for each branch its source-to-relay SNR is proportional to the (N+1)th power of its relay-to-destination SNR.

Optimal-Switching Adaptive Modulation for Multiuser Relay Networks with Feedback Delays

The performance of optimal switching adaptive modulation for multiuser amplify-and-forward (AF) relay networks over Nakagami-m fading channel is studied. Moreover, we consider two types of communication: i) relay-assisted communication (RAC): the source transmits its message to best user via relay path, ii) selective communication (SC): the source decides whether to forward the source message to the best user via relay path or direct path by comparing the end-to-end instantaneous signal-to-noise ratio (SNR) at the best user, which is independent of modulation scheme. Specifically, multiuser diversity and adaptive discrete-rate five-mode M-ary quadrature amplitude modulation (M-QAM) with constant transmit power is employed with fixed and optimal switching thresholds. The optimization criterion for switching thresholds is the maximization of spectral efficiency subject to target bit-error-rate. In particular, the detrimental effect of feedback delays in multiuser opportunistic scheduling (which exploits multiuser diversity) and adaptive modulation is quantified. To this end, an exact and an upper bound of the end-to-end SNRs are obtained and used to derive the cumulative distribution function and probability density function for each case. Moreover, exact outage probabilities, and lower bounds for the outage probabilities and average bit error rates, and the average spectral efficiencies are derived in closed-form. Further, we also present the high SNR approximations for outage probabilities and BERs. We observe that, for a particular spectral efficiency, adaptive five-mode M-QAM with optimal switching outperforms fixed switching and provides approximately 2 dB and 2.5 dB gain in average SNR for RAC and SC, respectively. Monte-Carlo simulations are performed to validate the analytical results.

Performance Evaluation of Adaptive Modulation System over Mobile Satellite Channels

The performance of an adaptive five-mode M-ary quadrature amplitude modulation (QAM) system under constant power constraint over time-varying land mobile satellite channel is evaluated in this paper. The mathematically-tractable closed-form expressions of performance for outage probability, bit error rate (BER) and average spectral efficiency are deduced by a simple Rician channel model in which the amplitude of the line-of-sight path is represented by the Nakagami distribution. Two adaptive modulation schemes (AMS) are compared, one with the optimal switching thresholds to maximize the average throughput under the constraint of average BER and another with fixed thresholds defined by the given BER tolerance for each modulation. Numeric simulation results show that the M -QAM AMS with optimum switching levels offers performance gains of ~2dB compared to fixed mode at given average spectrum efficiency, and nearly 0.3bps higher average throughput for different Rician K-factor.

On the Asymptotic Bit Error Probability of M-QAM for MIMO Y Channels

A theoretical analysis of bit error rate (BER) expression of adaptive M-ary quadrature amplitude modulation (M-QAM) for multiple-input multiple-output (MIMO) Y channels, where three users exchange independent messages with each other via a relay within two time slots, is proposed. Performance evaluation shows that the analytical BER closely matches the simulated one, particularly at high signal-to-noise ratios (SNRs).

Multiuser Diversity With Adaptive Modulation in Non-Identically Distributed Nakagami Fading Environments

In this paper, we analyze the performance of adaptive modulation with single-cell multiuser scheduling over independent but not identical distributed (i.n.i.d.) Nakagami fading channels. Closed-form expressions are derived for the average channel capacity, spectral efficiency, and bit error rate (BER) for both constant-power variable-rate and variable-power variable-rate uncoded/coded M-ary quadrature amplitude modulation (M-QAM) schemes. We also study the impact of time delay on the average BER of adaptive M-QAM. Selected numerical results show that the multiuser diversity brings considerably better performance, even over i.n.i.d. fading environments.

Performance analysis of adaptive M-QAM over a flat-fading Nakagami-<i>m</i> channel

Channel adaptive M-ary quadrature amplitude modulation (M-QAM) schemes have been developed to provide higher average link spectral efficiency by taking advantage of the timevarying nature of wireless fading channels. Much of the earlier work on such schemes uses the assumption that thresholds designed for additive white Gaussian noise (AWGN) channels can be directly applied to slowly varying block-fading channels. The thresholds are calculated with a commonly used approximation bit error rate (BER) expression in these schemes. The first aim of this paper was to investigate the accuracy of using this common BER expression in a fading channel. This was done by comparing the result of the average BER expression derived using the approximate expression with results of simulations over a Nakagami-m block-fading channel. The second aim was to show that the inaccuracy in the threshold values determined using the closed form approximation expression would lead to inappropriate operation of the adaptive M-QAM scheme in a fading channel. This was done by comparing expected theoretical values with the simulation results. Two alternative approximate BER expressions for M-QAM in AWGN were then presented and used to determine the average M-QAM of BER over a Nakagami-m fading channel. The comparison between the average BER expressions and the simulation showed a much closer match. More accurate thresholds for the adaptive M-QAM system were then determined using one of the two average BER expressions and the accuracy of these threshold points was then verified using simulation results.

A cross-layer design of wireless IP systems using effective bandwidth and MQAM adaptive modulation

In this paper, we introduce an effective bandwidth-based call admission control (CAC) with adaptive modulation technique to manage the traffic in a wireless IP-based network. Furthermore, in order to efficiently use the physical resources of the network, we take advantage of an adaptive MQAM (M-ary quadrature amplitude modulation) to match transmission rates to the time-varying channel conditions. This cross-layer architecture has been examined for the self-similar traffic model. The results show that by simultaneous implementation of the effective bandwidth algorithm in the data-link layer and adaptive modulation technique in the physical layer, the performance of the wireless network in terms of the number of rejected calls and system throughput improves.

Joint variable threshold M-QAM and ARQ for Nakagami-m fading channel with feedback delay

In this paper, we develop a cross-layer combining of adaptive M-ary quadrature amplitude modulation (M-QAM) and automatic repeat request (ARQ) protocol, which adapts the threshold of rate-region boundaries to the variation of feedback delay over the Nakagami-m fading (NMF) channels. We derive a threshold adaptation scheme to reduce the effect of the feedback delay on bit error rate (BER). Numerical results reveal that MQAM with threshold adaptation is more reliable to the feedback delay than M-QAM with fixed thresholds. When we combine the proposed scheme with ARQ, the enhanced BER can lower the probability of packet loss and reduce the number of packet retransmissions.

Joint Adaptive M-QAM and Selection Combining in SVD-Based MIMO Systems

In this paper, we propose a hybrid M-ary Quadrature Amplitude Modulation (M-QAM) transmission scheme that jointly uses discrete-rate adaptation and selection combining for singular value decomposition (SVD)-based multiple-input multiple-output (MIMO) systems, and derive exact closed-form expressions of the performance of the proposed scheme in terms of the average spectral efficiency and the outage probability.

Adaptive Modulation Based on Finite-Rate Feedback in Broadcast Channels

In a single-input single-output (SISO) downlink with multiple users, to obtain the multiuser diversity gain, the full channel state information (CSI) of all users is required for selecting the desired user and transmission mode. However, feedback channels are often capable of carrying only a limited number of bits. With such rate limited feedback links, we intend to maximize the average throughput of each user by designing transmit power levels, signal constellations and feedback thresholds jointly. Both un-coded adaptive M-ary quadrature amplitude modulation (MQAM) and adaptive trellis-coded modulation (TCM) are investigated. A nested iterative algorithm is proposed to obtain a finite number of transmission modes and thresholds. Optimization in the heterogeneous network is also considered. Simulation results demonstrate a small number of feedback bits make the throughput approach to that with perfect CSI. As the number of users increases, the multiuser diversity gain is achieved and the average feedback load of each user is reduced.

Slow Adaptive $M$-QAM With Diversity in Fast Fading and Shadowing

This paper investigates the performance of adaptive M-ary quadrature amplitude modulation (QAM) with antenna subset diversity. We consider a slow adaptive modulation (SAM) technique that adapts the constellation size to the slow variation of the channel due, for example, to shadowing. The proposed SAM technique is more practical than conventional fast adaptive modulation (FAM) techniques that require adaptation to fast-fading variations. Our results show that the SAM technique can provide a substantial increase in throughput with respect to fixed schemes while maintaining an acceptable low bit-error outage. We also compare SAM and FAM techniques, showing that the throughput of SAM can be, in many practical cases, close to that of FAM, despite the fact that SAM is less complex and requires a lower feedback rate. For example, using a set of possible modulations {4,16,64}-QAM with dual-branch maximal ratio combining reception, 5% outage at a bit-error probability of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and a median signal-to-noise ratio of 22 dB, SAM is capable of improving the mean spectral efficiency of fixed schemes from about 1.9 to 4.7 b/s/Hz, which is close to the 5.5 b/s/Hz achieved by FAM

Performance Evaluation of DVB-RCT Return Path in Fading Channels with added Diversity

The radio spectrum is a finite resource and it is important that all users exploit it efficiently. Consequently, modulation schemes used for the wireless environment should utilize the RF channel bandwidth, and the transmitted power, as efficiently as possible. Across Europe, the digital video broadcasting (DVB) group has proposed a technique for a high data-rate return path known as DVB-return channel through terrestrial (DVB-RCT) specifically for consumer device convergence (full Internet connectivity through a television set). The introduction of adaptive M-ary quadrature amplitude modulation (MQAM) into the DVB-RCT standards as opposed to static modulation (as used in the DVB-terrestrial (DVB-T) standard) has enhanced the performance of bidirectional communication. In this paper, the impact of using diversity techniques in such systems is investigated. In specific, the gain, obtained from utilizing this combination of adaptive MQAM and diversity techniques in the system, is also shown.

Adaptive coded modulation for fading channels

We apply coset codes to adaptive modulation in fading channels. Adaptive modulation is a powerful technique to improve the energy efficiency and increase the data rate over a fading channel. Coset codes are a natural choice to use with adaptive modulation since the channel coding and modulation designs are separable. Therefore, trellis and lattice codes designed for additive white Gaussian noise (AWGN) channels can be superimposed on adaptive modulation for fading channels, with the same approximate coding gains. We first describe the methodology for combining coset codes with a general class of adaptive modulation techniques. We then apply this methodology to a spectrally efficient adaptive M-ary quadrature amplitude modulation (MQAM) to obtain trellis-coded adaptive MQAM. We present analytical and simulation results for this design which show an effective coding gain of 3 dB relative to uncoded adaptive MQAM for a simple four-state trellis code, and an effective 3.6-dB coding gain for an eight-state trellis code. More complex trellis codes are shown to achieve higher gains. We also compare the performance of trellis-coded adaptive MQAM to that of coded modulation with built-in time diversity and fixed-rate modulation. The adaptive method exhibits a power savings of up to 20 dB.